Semiconductor integrated circuits are typically fabricated on a wafer or substrate of a semiconductor material such as, for example, silicon or gallium arsenide. During the fabrication, the wafer is subjected to a sequence of steps, which may include photomasking, material deposition, oxidation, nitridization, ion implantation, diffusion, and etching, among others.
Etching may be achieved by wet etching processes or dry etching processes. Dry etch processes, such as a plasma etch or ion-assisted etch, are known for etching materials for semiconductor fabrication in silicon integrated circuit technology. Plasma etches are largely anisotropic or unidirectional. Plasma etches may be used to create spaces or substantially vertical sidewalls in the integrated circuit layers, to transfer a mask pattern to an underlying layer with little or no undercutting beneath mask segment edges and to create contact paths in insulative layers. Plasma etch processes are especially useful for producing sub-quarter micrometer patterns and geometries.
Semiconductor integrated circuits with high device density require the patterning of closely spaced submicrometer lines in semiconductors materials to form submicron geometries such as small area emitters for bipolar transistors, short gates for field effect transistors and narrow interconnection lines between devices. The formation of such polysilicon, metal or insulator structures typically requires definition of the locations of such structures in a layer of photoresist on a layer of polysilicon or insulator by exposure of the photoresist with light passing through a reticle or photomask containing the desired pattern. After exposure and treatment of the photoresist, the underlying layer of the substrate is plasma etched using the patterned photoresist as a template. The masking material protects designated areas of the substrate from the etch process. Subsequent processing steps are determined according to the type of device to be fabricated.
As advances in photolithographic and processing capabilities progressively increase, the lateral dimensions of features in silicon integrated circuits continue to decrease. Fabrication of reduced device geometries in integrated circuits mandates minute contact holes of submicron size on insulation layers and minimum isolation distance requirements measured in terms of critical dimensions (CD). For example, recent generations of complementary metal-oxide silicon integrated circuits (CMOS) have gate regions with dimensions on the order of 0.25 microns, or even 0.18 microns and less in the near future.
As the integrated circuit manufacture goes to the sub-quarter regime, a challenge to the high aspect ratio is that the deep ultraviolet (DUV) resist needed to pattern the integrated circuit is thinner and more malleable than prior photoresists. Large striations and uncontrolled increases in the size of the contact holes, known as CD losses, are common during the photolithographic process in the sub-quarter micron regime.
During photolithography, problems arise because high resolution submicrometer images in photoresist require shallow depth of focus during exposure, but thick photoresist patterns are required because of the poor etch rate between the photoresist and the underlying semiconductor layer. Additional problems occur because of the uncontrolled bake during the plasma etch processing. During this process, the substrate is exposed to ion and electron bombardment, UV light, X-rays, and scattered radiations. As a consequence, irregular topographies, distorted images and CD loss occurs during the exposure of the photoresist layer as shown in FIGS. 1-2. These figures illustrate a typical plasma etch of a silicon substrate 40 having an oxide layer 42 deposited thereon. Contact holes 10, 12, 14 are etched into wafer 10. The contact holes 10, 12, 14 have an upper surface 38 and a lower surface 36. Due, in part, to the thin DUV resist and the uncontrolled bake during the etching process, discontinuities 18, 20, 22, 24, 26, 28, 30 and 46 are formed as shown for contact hole 10. The discontinuities 18, 20, 22, 24, 26, 28, 30, 46 occur in the contact hole 12, 14 as a result of the plasma etch attacking the side walls of the contact holes 12, 14. It should be understood that the shape and number of the discontinuities will vary depending upon the specific etching process parameters as well as the material which is being etched. The discontinuities may form which have a first surface 32 and a second surface 34 in the wall 44 of the contact hole 12. In addition, contact holes 10, 12, 14 are formed in a frusto-conical shape instead of a cylindrical shape when formed in the oxide layer 42.
When two discontinuities 22, 46 are formed in adjacent contact holes 12, 14 and become aligned with one another, the integrated circuit suffers a loss in critical dimension (CD loss). CD loss is a critical component of integrated circuit design, especially in the sub-quarter micron regime. Additionally, when the contact holes 10, 12, 14 are formed in a frusto-conical shape instead of the desired cylindrical shape, surface area is sacrificed thereby requiring the contact holes 10, 12, 14 to be deeper to effectuate the same contact.
A further problem with the prior plasma etching is that as a result of the irregular contact holes 10, 12, 14, an unwanted and uncontrolled increase in the diameter of the contact holes 10, 12, 14 may also result. This increased size also impacts the displacement of the metal atoms that fill the contact holes. Thus, in addition to the loss in critical dimension, electrical contacts may also become unreliable.
Several attempts have been made to solve this problem. It has been suggested that the distorted images can be alleviated by employing a three-layer photoresist technique such as in U.S. Pat. No. 5,242,532 (Cain) or by employing a silylation layer process such as in U.S. Pat. No. 5,312,717 (Sachdev et al.). These solutions, however, require additional time consuming and costly steps in the etching process.
Accordingly, there is a need for improved plasma etching that provides a substantially uniform etch without a reduction in the critical dimension and without striations formed in the sidewalls of the etched portion of the substrate. The improved plasma etching technique should provide a substrate having increased uniformity across the substrate surface, a substantially uniform trench, a substantially uniform profile angle and a smooth sidewall.